The present invention relates to radiation shields used in superconducting magnet cryostats.
The construction of a low cryogen loss superconducting magnet cryostat generally requires the use of at least two radiation shields between a liquid helium vessel and ambient. Typically, the warmer shield will be liquid nitrogen cooled and the colder shield will be helium boil off gas cooled. The colder shield, referred to as the intermediate shield, can be supported by tie rods serving as tension members from the nitrogen vessel, which is in turn supported by tie rods from the ambient. Tie rods from the nitrogen vessel also support the helium vessel. The tie rod method of support requires that the shield be sufficiently rigid so that the space between the intermediate shield and the other vessels can be kept to a minimum without danger of thermal shorts developing (heat transfer does not benefit from increased spacing in a vacuum). An example of this type construction is shown in U.S. Pat. No. 4,492,090, entitled "Cryostat For NMR Magnet".
Long tension supports are standard practice for high performance cryostats; titanium and glass fiber composites are commonly used materials. In vertical dewars, intermediate radiation shields can be supported by a single connection on the dewar neck, an option not available for the horizontal magnetic resonance magnets.
The highest performance insulation systems currently available consist of some type of multilayer insulation. Application of multilayer insulation is labor intensive and cannot ordinarily be justified for use at liquid helium temperatures. Also, relatively small errors in the application of the insulation can substantially reduce its effectiveness compared to hard vacuum. Multilayer insulations are widely used and assume various forms including dimpled or crinkled foil layers and foil or aluminized plastic, separated with glass fiber mats. Typical application to the annular shaped vessels in an magnetic resonance cryostat requires wrapping on the outside diameter of the vessel, wrapping on the inside of the vessel and then interweaving the wrapped layers with "doughnut" shaped layers on the end caps. Since wrapping on the inside diameter is not possible, the inside diameter wrapping must be made on a form and held in place until the interweaving operation is complete.
It is an object of the present invention to eliminate tie rod support of the intermediate shield in the cryostat of a superconducting magnet thereby reducing the complexity of the nitrogen vessel and simplifying cryostat assembly.